1. Field of the Invention
The present invention relates to the field of electronic circuitry for display devices. More specifically, the present invention relates to the field of display device technology that automatically detects and responds to video signals of differing video formats.
2. Related Art
Cathode ray tube (CRT) display devices have been used in conjunction with computer systems for many years. CRT display devices display information to computer users in the form of graphic images. Basic to all CRT displays is a cathode ray tube including a screen having phosphor disposed thereon. An electron gun is used that emits an electronic beam which is directed toward the phosphor thereby causing the emission of light. The electron beam is controlled by deflection units that use magnetic fields to sweep the electron beam across the screen in horizontal lines from top to bottom traversing the vertical dimension of the screen (e.g., to display a field or frame of information). Frames are typically presented on the screen at rates of between 40-60 Hz. Vertical and horizontal timing pulses synchronize the electron beam. The emission of electrons from the electron gun is controlled by color signals (e.g., read, green, blue).
Computer systems generate video signals (e.g., RGB and synchronization signals) that are used to control a CRT display device. There are currently many different video signal formats that computer systems use and each video signal format offers different numbers of pixels along the horizontal and vertical dimensions of the screen. For instance, one video signal format divides the screen into 640 pixels along the horizontal and 480 pixels along the vertical (e.g., "640.times.480"). Another popular video signal format divides the screen into 1024 pixels along the horizontal and 768 pixels along the vertical (e.g., "1024.times.768"). In the past, the electronics of a given CRT display device was manufactured to be compatible with only one video signal format. Therefore, the computer system's video format and the CRT display device's video format needed to match exactly for proper operation and image quality.
However, with the emergence of many video formats in the commercial market for computer systems, and with today's computer systems being able to support multiple video signal formats, display manufactures have responded by developing a CRT display device that is automatically compatible with several different video signal formats. Recently, CRT display devices have been introduced that automatically detect and respond to several different video signal formats without user intervention. These CRT display devices contain circuitry that performs "automatic sizing and centering" functions to properly locate the frames on the display screen regardless of the video signal format (within some range of accepted and standardized video signal formats)
FIG. 1A illustrates circuitry used within one such prior art CRT display device 10a that performs automatic sizing and centering functions thereby making it compatible with multiple video signal formats. The CRT display device 10a includes at least two PC boards, a first board 12a and a second board 14a. The first board 12a receives video color signals 16 and synchronization signals 18 and 20 (e.g., originated from a video card of a host computer system). A video receiver circuit 30 is used to amplify the video color signals 16 and supply them to an OR circuit 32 which supplies an OR signal to a microprocessor unit 34. The microprocessor unit 34 compares this OR signal with the synchronization signals 18 and 20 and generates sizing signals 22 and centering signals 24 which are sent to the second board 14. The second board 14a includes a yoke control device 36 which uses the sizing 22 and centering signals 24 to control the deflection of the electron beam within a cathode ray tube 40. The microprocessor unit 34 performs automatic sizing and centering functions so that the CRT device 10a can automatically detect and respond to different video signal formats.
One problem with the design 10a of FIG. 1A is that the first board 12a needs to be closely located to the second board 14a due to noise and signal jitter issues with respect to the analog sizing 22 and centering signals 24. Also EMI radiation from the analog video signal (signals 16, 18 and 20) is a problem as is signal delay from the first board 12a to the second board 14a. However, design flexibility of the overall circuit 10a is reduced if the first board 12a is required to be tightly positioned with respect to the second board 14a. One method for reducing the noise problem is to use costly EMI protection covering. However, this solution is not commercially advantageous due to the additional cost required for shielding and also because this solution does not solve the underlying signal delay problems. Therefore, it would be advantageous to reduce or eliminate the noise and signal delay problems referenced above to obviate any requirement that the first board 12a be closely positioned with respect to the second board 14a.
Another problem associated with the design 10a of FIG. 1A is that once the position of the microprocessor 34 is fixed, it becomes very difficult to optimize the layout of the remainder of the components because a great deal of attention needs to be paid to noise, signal delay and EMI susceptibility. For instance, generally once the position of the large microprocessor 34 is fixed, it is not always guaranteed to be near the video receiver 30, thereby causing noise and jitter problems associated with the transmission lines between the input video signal and the microprocessor 34. It would be advantageous to provide an automatic sizing and centering design that has reduced susceptibility to noise, signal delay and EMI problems. Another problem with the design 10a of FIG. 1A is that the microprocessor 34 typically operates at a lower frequency (e.g., 32 MHz) with respect to the input video signal. Lower frequency translates into lower accuracy of the automatic sizing and centering functions.
FIG. 1B illustrates circuitry used within another prior art CRT display device 10b that performs automatic sizing and centering functions like circuit 10a thereby making it compatible with multiple video signal formats. The CRT display device 10b also includes at least two PC boards, a first board 12b and a second board 14b. However, in circuit 10b, the microprocessor unit 34 is included on first board 12b. This prior art circuit 10b also has EMI issues and signal delay issues, as discussed above, with respect to line 46. Line 46 connects between the first board 12b and the second board 14b. In view of the above problems, it would be advantageous to provide automatic sizing and centering functionality with increased accuracy.